Re: [PATCH 2/4] drm/uAPI: Add "active bpc" as feedback channel for "max bpc" drm property

[Pekka Paalanen]

On Fri,  4 Jun 2021 19:17:21 +0200
Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx> wrote:

> Add a new general drm property "active bpc" which can be used by graphic drivers
> to report the applied bit depth per pixel back to userspace.
> 
> While "max bpc" can be used to change the color depth, there was no way to check
> which one actually got used. While in theory the driver chooses the best/highest
> color depth within the max bpc setting a user might not be fully aware what his
> hardware is or isn't capable off. This is meant as a quick way to double check
> the setup.
> 
> In the future, automatic color calibration for screens might also depend on this
> information available.
> 
> Signed-off-by: Werner Sembach <wse@xxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_atomic_uapi.c |  2 ++
>  drivers/gpu/drm/drm_connector.c   | 40 +++++++++++++++++++++++++++++++
>  include/drm/drm_connector.h       | 15 ++++++++++++
>  3 files changed, 57 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
> index 268bb69c2e2f..7ae4e40936b5 100644
> --- a/drivers/gpu/drm/drm_atomic_uapi.c
> +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> @@ -873,6 +873,8 @@ drm_atomic_connector_get_property(struct drm_connector *connector,
>  		*val = 0;
>  	} else if (property == connector->max_bpc_property) {
>  		*val = state->max_requested_bpc;
> +	} else if (property == connector->active_bpc_property) {
> +		*val = state->active_bpc;
>  	} else if (connector->funcs->atomic_get_property) {
>  		return connector->funcs->atomic_get_property(connector,
>  				state, property, val);
> diff --git a/drivers/gpu/drm/drm_connector.c b/drivers/gpu/drm/drm_connector.c
> index 7631f76e7f34..5f42a5be5822 100644
> --- a/drivers/gpu/drm/drm_connector.c
> +++ b/drivers/gpu/drm/drm_connector.c
> @@ -1195,6 +1195,13 @@ static const struct drm_prop_enum_list dp_colorspaces[] = {
>   *	drm_connector_attach_max_bpc_property() to create and attach the
>   *	property to the connector during initialization.
>   *
> + * active bpc:
> + *	This read-only range property is used by userspace check the bit depth
> + *	actually applied by the GPU driver after evaluation all hardware
> + *	capabilities and max bpc. Drivers to use the function
> + *	drm_connector_attach_active_bpc_property() to create and attach the
> + *	property to the connector during initialization.
> + *

Hi Werner,

the idea looks good to me, but the above doc could be a little more
fluent. May I suggest something like:

	This read-only range property tells userspace the pixel color
	bit depth actually used by the hardware display engine on "the
	cable" on a connector. The chosen value depends on hardware
	capabilities, both display engine and connected monitor, and
	the "max bpc" property. Drivers shall use
	drm_connector_attach_active_bpc_property() to install this
	property.

There should also be something said about dithering done by the display
engine (not monitor), but I'm not sure how that should be worded. It
may also depend on if and how userspace can know about dithering. So if
a dithering related property is added later, maybe add a note here too
in that patch.


Thanks,
pq


>   * Connectors also have one standardized atomic property:
>   *
>   * CRTC_ID:
> @@ -2150,6 +2157,39 @@ int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>  }
>  EXPORT_SYMBOL(drm_connector_attach_max_bpc_property);
>  
> +/**
> + * drm_connector_attach_active_bpc_property - attach "active bpc" property
> + * @connector: connector to attach active bpc property on.
> + * @min: The minimum bit depth supported by the connector.
> + * @max: The maximum bit depth supported by the connector.
> + *
> + * This is used to check the applied bit depth on a connector.
> + *
> + * Returns:
> + * Zero on success, negative errno on failure.
> + */
> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector,
> +					  int min, int max)
> +{
> +	struct drm_device *dev = connector->dev;
> +	struct drm_property *prop;
> +
> +	prop = connector->active_bpc_property;
> +	if (!prop) {
> +		prop = drm_property_create_range(dev, 0, "active bpc", min, max);
> +		if (!prop)
> +			return -ENOMEM;
> +
> +		connector->active_bpc_property = prop;
> +	}
> +
> +	drm_object_attach_property(&connector->base, prop, 0);
> +	connector->state->active_bpc = 0;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL(drm_connector_attach_active_bpc_property);
> +
>  /**
>   * drm_connector_set_vrr_capable_property - sets the variable refresh rate
>   * capable property for a connector
> diff --git a/include/drm/drm_connector.h b/include/drm/drm_connector.h
> index 1922b278ffad..c58cba2b6afe 100644
> --- a/include/drm/drm_connector.h
> +++ b/include/drm/drm_connector.h
> @@ -781,6 +781,13 @@ struct drm_connector_state {
>  	 */
>  	u8 max_bpc;
>  
> +	/**
> +	 * @active_bpc: Read only property set by the GPU driver to the actually
> +	 * applied bit depth of the pixels after evaluating all hardware
> +	 * limitations.
> +	 */
> +	u8 active_bpc;
> +
>  	/**
>  	 * @hdr_output_metadata:
>  	 * DRM blob property for HDR output metadata
> @@ -1380,6 +1387,12 @@ struct drm_connector {
>  	 */
>  	struct drm_property *max_bpc_property;
>  
> +	/**
> +	 * @active_bpc_property: Default connector property for the active bpc
> +	 * to be driven out of the connector.
> +	 */
> +	struct drm_property *active_bpc_property;
> +
>  #define DRM_CONNECTOR_POLL_HPD (1 << 0)
>  #define DRM_CONNECTOR_POLL_CONNECT (1 << 1)
>  #define DRM_CONNECTOR_POLL_DISCONNECT (1 << 2)
> @@ -1698,6 +1711,8 @@ int drm_connector_set_panel_orientation_with_quirk(
>  	int width, int height);
>  int drm_connector_attach_max_bpc_property(struct drm_connector *connector,
>  					  int min, int max);
> +int drm_connector_attach_active_bpc_property(struct drm_connector *connector,
> +					  int min, int max);
>  
>  /**
>   * struct drm_tile_group - Tile group metadata

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